Development of a Magnetostrophic Geodynamo

发电机是一种将机械能转化为电能的装置。它通过移动一个电导体穿过磁场来实现这一点。这会产生电流，从而产生自己的感应磁场。当感应磁场和感应磁场相同时，发电机变为自励。类似的自激过程产生了地球的磁场。 相关的电流在地核中流动，地核是一个近似球形的液态铁，半径为2,200英里，围绕着地球的中心。由于这种导电流体的运动，地球的核心是一个巨大的发电机，称为“地球发电机”。发电机也需要一个电源来维持其运动和发电，尽管通过电阻损失热量。对于地球发电机来说，动力来源被认为是由于地核中密度差异的浮力。功率不仅用于维持磁场，而且用于克服与流体运动相反的摩擦力。目前的数值模拟假设摩擦能量损失与电阻相当，但人们普遍认为，实际上摩擦能量损失要小得多。忽略摩擦损失的发电机被称为磁致旋转发电机，研究人员最近恢复了等离子体物理学家布莱恩·泰勒（Bryan Taylor）的一个旧想法，这使他们采用了一种新的数值方法，通过这种方法，他们已经导出了磁致转动方程的二维解。研究人员相信，当他们把这些解变成三维的（这是一个复杂的步骤）时，它们将成为未来模拟磁场产生的新范例，并有助于更好地理解地球和其他行星是如何产生磁性的。磁罗盘指针的寻北特性表明，科里奥利力强烈地影响着地核运动。这种力主要由浮力和洛伦兹力平衡，洛伦兹力是磁场施加在载流导体上的力。泰勒在1953年展示了如何实现由此产生的简化的动力平衡，因此确定了流体速度作为磁场的函数。 为了闭合再生回路，有必要根据流体速度确定磁场，但这是众所周知的运动发电机过程。在1953年，前进的道路似乎很简单，但自那时以来，在今年调查小组之前，为实施泰勒的建议所做的所有坚定的尝试都失败了。 通过假设相对于地球自转轴的对称性，该模型被大大简化。 众所周知的阿尔法效应被用作动力来源。这也使得考林定理，否则将排除轴对称的解决方案，不适用。 为了让发电机社区相信这一成功是有意义的，该团队计划放弃轴对称，消除α效应，并依靠浮力来提供所需的动力。 研究人员认为，他们的结果将为发电机社区大开眼界，并将成为行星和恒星中自然发生的发电机模型的基础，这些模型比今天的模拟更现实，假设粘性摩擦非常大。

A dynamo is a device that converts mechanical energy into electrical energy. It does this by moving an electrical conductor across a magnetic field. This induces an electrical current which creates its own induced magnetic field. The dynamo becomes self-excited when the induced and inducing magnetic fields are the same. A similar self-excited process creates Earth's magnetic field. The associated electric currents flow in the Earth's core, a nearly spherical mass of liquid iron, of radius 2,200 miles, surrounding the Earth's center. Because of this conducting fluid's motion, the Earth's core is a gigantic electric generator, called 'the geodynamo'. A dynamo also needs a power source to maintain its motion and production of electricity despite heat losses through electrical resistance. For the geodynamo the power source is thought to be due to the buoyancy of density differences in the core. The power is expended not only in maintaining the magnetic field but also in overcoming the frictional forces opposing the fluid motion. Current numerical simulations assume that the frictional energy losses are comparable with those from electrical resistance, but it is generally agreed that in reality they are very much less. A dynamo that ignores frictional losses is termed magnetostrophic, and the investigators have recently revived an old idea of the plasma physicist, Bryan Taylor, that has led them to a new numerical procedure through which they have already derived two-dimensional solutions of the magnetostrophic equations. The investigators believe that, when they have made the solutions three-dimensional (a complicated step), they will become the new paradigm for future simulations of magnetic field creation and lead to a better understanding of how the Earth and other planets create their magnetism.The north-seeking property of the magnetic compass needle shows that the Coriolis force strongly influences core motion. This force is balanced primarily by buoyancy and the Lorentz force, which is the force exerted on a current-carrying conductor by the magnetic field. Taylor showed in 1953 how the resulting simplified dynamical balance could be achieved, so determining the fluid velocity as a functional of the magnetic field. To close the regenerative loop, it is necessary to determine the magnetic field from the fluid velocity, but that is the well-known and well-understood kinematic dynamo process. The way ahead seemed simple in 1953 but all the determined attempts made since then to implement Taylor's suggestion failed, prior to that of the investigating team this year. The model was greatly simplified by assuming symmetry with respect to the Earth's rotation axis. The well-known alpha-effect was invoked as the power source. This also made Cowling's theorem, that otherwise would have ruled out axisymmetric solutions, inapplicable. To convince the dynamo community that this success is meaningful, the team plans to abandon axisymmetry, remove the alpha-effect, and rely on buoyancy to supply the required power. The investigators believe that their results will be an eye-opener for the dynamo community, and will become the basis for models of naturally-occurring dynamos in planets and stars that are more realistic than today's simulations which assume a viscous friction that is very much too large.